@Acolytus
You're right, I forgot about that!
The phrasing does not imply that there is any delay between the enthronement and the initiation of the war so that would indeed imply that the pacifist Demon Lord was replaced and his successor launched the war. (I can't recall, but might there have been a mention in one of the chapters that the current Demon Lord, who happens to be a pacifist, has reigned for some time already?) Who knows how the successor is related to the current Demon Lord and what motivations he had when starting the war but, in any case, it appears he had some regrets with how events unfolded; they do end in his death after all.
I'm unsure the warmonger Demon Lord had evil intentions yet; he may have been manipulated as he still had the support of the dragons and the Hero is under the belief that the demon's army movement through the human kingdoms had to have been facilitated by the merchant, so that merchant still supported a Demon Lord intent on conquering human nations and causing destruction with little care for loss of life? That said, the merchant relations could have been abused, and the dragons manipulated into fighting.
In any case, the Hero meeting the current Demon Lord means he might also meet the next one if he happens to be in his entourage. I doubt he will come clean about his mind having traveled back in time yet while he ascertains their intentions so we may not learn a whole lot.
I sure wish the chapters came out more often.